This invention relates to commutation control of a dynamoelectric machine such as a poly-phase electric motor used in household or similar appliances and, more particularly, to an improved sensorless controller for providing commutation control.
In U.S. patent application 08/004411, filed Jan. 14, 1993, and assigned to the same assignee as the present invention, there is described a method and apparatus for achieving commutation control of a dynamoelectric machine. The particular advantage of the invention described therein was that, unlike prior control methods and circuitry, there was no reliance on physical sensing elements such as Hall effect sensors to obtain the commutation information needed to determine the relative position of the machine's rotor and stator windings. Commutation angle corrections, if necessary, can be made. Rather, the method and apparatus of that invention involved sampling the DC bus current waveform and analyzing the samples to determine if there were an "in-phase", leading, or lagging commutation angle. If the commutation angle was determined to be leading or lagging as a result of the analysis, appropriate correction was made to adjust it to be "in-phase" or to a desired commutation angle for the particular set of machine operating conditions. The method recognized that as operating conditions changed, so would the desired "in-phase" relationship between the rotor and stator windings. Further, the invention allowed for commutation control throughout the entire range of machine operating speeds, rather than the limited speed range of control achievable with prior commutation controllers.
While generally effective, it has been found that in some instances, the commutation angle control analysis is effected by bus ripple, transients on the bus, and similar phenomena. The motor with which the commutation apparatus is used, typically includes a filter capacitor for filtering out ripple and transient effects on the bus line. However, at higher power applications, the capacitor may be unable to completely filter out these AC components. As a result, the current waveshape is distorted by the ripple and the transients and the desired level of control is lost. In applications where there is no line filter (capacitors, or inductors, or both), line AC components will be present on the bus.
In the sampling scheme disclosed in the 08/004411 application, now U.S. Pat. No. 5,420,492, the current envelope is sampled at two points during each commutation period. The sample values are then combined to produce a ratio value from which the commutation relationship between the rotor and stator phase windings is determined. Because of the envelope distortion produced by these AC and transient effects, or other system instabilities, the waveshape may be distorted such that the processed samples inaccurately reflect the phase relationship. The subsequent control applied to produce an "in phase" relationship for this condition now introduces a degree of instability into the motor/controller because the current waveshape (and the resultant ratios) vary widely from one commutation interval to the next. This situation could be corrected were it possible to fully eliminate the ripple and transient effects on the current envelope; or, compensate for their effects during processing of information obtained from waveshape sampling.